In hyperbolic navigation systems such as LORAN and OMEGA, the utilization of real time plotting as an aid to navigation has existed in the past in which time differences of arrival of signals from a master and slave stations indicate the position of the vessel or vehicle receiving the signals. It is the major function of these plotters to plot the track of the vessel or vehicle on a plotting surface so as to achieve a permanent record of the vehicle's progress. In terms of commercial fishing, it is often times necessary to display the area that a vessel traverses over a pre-determined fishing ground so that the pilot of the vessel may more accurately control his vessel for complete coverage.
Navigational plotting systems such as that described are also useful in air-sea search in terms of giving the navigator a clear picture of the area searched so that completeness of the search may be ascertained as well as the prevention of duplication of efforts.
The navigational plotter also creates a permanent record of the track of the vehicle so that proximity to navigational hazards may be readily ascertained and so that progress to a given point, harbor or place of refuge may be quickly displayed.
Some of the problems which have existed in prior plotters, are the need for the superposition of hyperbolic time difference lines on the charting surface for those charts which are not already over-printed with hyperbolic lines.
It will be appreciated that in the past the drawing of hyperbolic lines has been a time-consuming process even utilizing computer driven X-Y plotting devices. Thus on-station printing of hyperbolic lines has been both inconvenient and in times of emergency, not rapid enough to provide adequate navigation safety.
Concomitant with the drawing of hyperbolic lines is the necessity for initializing the system both as to the scale of the chart or charting surface to be utilized and as to the expected microsecond spacing of the hyperbolic lines in the area of interest. Moreover, since sets of hyperbolic time difference lines cross at more than one position on the earth's surface (with one crossing in an "extended area") it is usually necessary to manually enter the vessel's estimated latitude and longitude before a plotter can use the hyperbolic time differences (TDs) and convert them into latitude and longitude (lat./long.) for driving a plotting pen. This precludes total automatic operation. Also, it is important to be able to take into account the local anomalies and to be able to correct the plotter for a variety of different conditions.
It will be appreciated that merely placing the pen on a plotting surface and driving it in accordance with hyperbolic navigation signals is insufficient to create a true record of either vehicle position or vehicle track both as regards to fixed positions on the chart such as navigational channels and navigational aids and as to the scale itself. Additionally, it is important to be able to reference the pen position on the chart to known positions of objects on a chart such as a buoy or a rock, the latitude and longitude of which is precisely known. Without this type of initialization, scribings on the chart may not only be off-location, and therefore misrepresent the position of the vessel, but also differences in scale will obviously distort the vehicle track.
Although unknown to many mariners, in any hyperbolic navigation system, there is a danger zone which exists about the extension of the base line between the master station and a slave station in that there is an ambiguity as to exact position. For instance, when crossing such a zone, there are two points at which the same hyperbolic navigation reading is obtained and unless the navigator is aware that such a condition exists, he may infer a different position from his actual position. This extended base line danger zone is characteristic of all hyperbolic navigation systems and since the location of the slaves and master stations are usually unknown to the navigator, it is impossible for him to determine the existence of danger zones.
With respect to the actual plotting of the vehicle's track, it will be appreciated that there exist certain transients in signal detection which give false readings of positions. This can result in the vehicle track being deflected by large amounts which is not only disconcerting, but also often times the result is a misreading of position.
Of course, due to the limited charging area of the plotter, there are many occasions when the actual vehicle or vessel location is off-chart. While in the past there have been various techniques of stopping pen movement until the vehicle or vessel arrives back onto the chart surface, often times it takes an inordinately long time for the internal pen-drive and position indicating counters of the plotter to count up to the off-chart position. Thus all calculations must wait until the counters are clocked to the correct number, a time period which may be in excess of five minutes if, as is usual, the counters are clocked at the pen-drive stepping motor rate.
Another off-chart situation which results in a slow machine is the way in which hyperbolic lines are drawn. In one method of approximating a true hyperbolic line and as will be discussed hereinafter, a START point is defined on the true line and a GOAL is defined on the true line. If the deviation of the proposed line between START and GOAL from the true hyperbolic line is below a certain limit, a line is drawn which commences at the START point and ends up at the GOAL. Depending on the curvature of the true line, drawing lines in this manner may be a time consuming process involving drawing of a large number of short stubby lines.
Moreover, if the GOAL is off the chart, it is necessary to count up the counters to this GOAL point. If this is done at the pen-drive rate, it takes a long time to count up the counters.
Additionally, in repositioning the pen for the drawing of the next hyperbolic line, the pen repositioning must wait until the appropriate counters are either counted up or counted down. This traditionally has been done at the pen-drive rate.